1. Field of the Invention
This invention relates in general to computer graphics display systems and more particularly to a processing method and apparatus for identifying a displayed object that intersects an operator selected area of the display screen. The invention more particularly relates to detection of an operator selection prior to detailed rendering of the drawn figure.
2. Description of the Prior Art
Interactive raster graphics systems, such as CAD/CAM workstations, are widely used to design components and systems of mechanical, electrical, electromechanical, and electronics devices. Frequently, the emphasis within such systems is on the operator interaction with a computer based model of a component or system being designed in order to modify or test that component. The computer based model is comprised of numerous graphics objects that are individually processed and displayed for operator action. Selection of the displayed object is accomplished via any one of a number of operator controlled interaction devices such as light pens, locators (e.g., a data tablet with stylus), and alphanumeric and function keyboards. An important part of many interaction sequences is computer identification of an operator selected displayed object to be operated upon, a process known as picking.
Typically, a graphics system processing technique requires the reprocessing of an entire display screen to identify the particular displayed object selected or picked for further processing. More particularly, existing processing methods require the re-execution of the entire display program, including: transformation of each geometric primitive defining a displayed object in world coordinate space; clipping of each transformed primitive against the predefined clipping boundary in world coordinate space; mapping of each clipping primitive to an operator defined viewport and screen coordinate space; rasterization of all mapped data; and finally, determination of whether the generated pixels intersect the operator defined selection area in a window in screen coordinate space. If so, then a pick occurs.
Although effective, a clear drawback to this procedure is the unnecessary processing occurring as a result of mapping and rasterization geometric objects not selected. For example, drawing or rasterization of a filled polygon outside the operator selected screen area is both unnecessary and time consuming.
One attempt to address this problem is discussed in "Interactive Computer Graphics Display Screen Processing Method for Identifying an Operator Selected Displayed Object", U.S. application Ser. No. 299,901, filed Jan. 23, 1989, now U.S. Pat. No. 4,982,345 and commonly assigned. That patent discusses apparatus and method for forming an inverse mapping of the pick window into world coordinate space. This technique allows picking to occur in world coordinate space prior to window to viewport mapping from three dimensional world coordinate space to two dimensional screens. While this technique provides a method to pick earlier in the pipeline cycle and therefore avoid rasterizing objects unnecessarily, it suffers from certain disadvantages. Most notable are the problems encountered when trying to inversely map a two dimensional pick window into three dimensional world coordinate space. Advanced techniques used to map from three to two dimension, such as the use of perspectives, or other selectable features, makes precise inverse mapping of the pick window difficult.
Therefore, an improved apparatus and method for pick detection prior to rasterization and without the limitations of inversely mapped pick windows is needed.